Shigella species are Gram-negative, facultative intracellular enteric bacteria that specifically infect and cause disease in the large intestine of human hosts. Shigella spp. are the causative agent of bacillary dysentery (shigellosis) and the majority of infections occur in countries with inadequate health care and unsafe food and water supplies; however. Shigella outbreaks are a significant problem for industrialized nations as well. The signature feature of this disease is exhibited by an intense inflammatory reaction manifested clinically as passage of bloody stools. This project focuses on novel strategies used by S. flexneri to promote tissue invasion, which elicits the inflammatory response leading to PMN recruitment. Contrary to current dogma, we hypothesize that speciflc bacteria-host interactions that occur at the apical epithelial surface of I EC are critical for Shigella pathogenesis. To test this hypothesis. Specific Aim 1 will dissect the molecular mechanisms by which S. flexneri regulates the tight junction proteins. The goal of this aim will be to determine the S. flexneri genes that regulate the specific tight junction proteins ZO-1 and ZO-2. In Specific Aim 2 we will determine how the Shigella secreted protein, SepA, promotes tissue invasion and disease pathogensis from the apical surface: SepA is the major protein secreted by S. flexneri (in vitro), and is an autotransporter serine protease encoded by the virulence plasmid. This aim will examine important interactions that occur between SepA and the lEC apical surface, which are critical for the ability of S. flexneri to elicit a pro-inflammatory response leading to the recruitment of PMN. Finally, in Specific Aim 3, we will determine the molecular mechanism by which S. flexneri governs HXA3 efflux from the apical membrane domain during S.flexneri infection. We speculate that HXAS is required for neutrophilic breach of the intestinal epithelium during S. flexneri infection. Therefore, understanding the molecular mechanisms that orchestrate the apical release of HXAS will be crucial to designing improved therapies to augment host defense and attenuate detrimental intestinal inflammation during Shigella infection. Ultimately, understanding the molecular mechanisms of S. flexneri entry and induction of pro-inflammatory responses will provide to new insights for controlling the massive inflammatory pathology underiying shigellosis.